Synthetic imaging systems are known by the acronym “SVS”, standing for “Synthetic Vision System”. Real imaging systems are known by the acronym “EVS”, standing for “Enhanced Vision System”. The sensors used are, for example, infrared sensors, millimeter wave radars or else low-light-level sensors.
The combination of the two systems is called “CVS”, standing for “Combined Vision System”. “CVS” imaging can be displayed on a screen in “head-down” mode or a “head-up” viewing device, worn or not by the user.
SVS imaging considerably enhances the awareness of the situation of the crew by displaying an image of the outside scene that is independent of the meteorological conditions. But the inaccuracies of satellite location and/or the lack of integrity of the databases do not allow this system to be adequate for its use during low-altitude flight or landing. Its use is therefore relevant for displaying terrain relatively distant from the aircraft.
An enhanced vision system EVS is an electronic means for providing an image of the outside scene that is enhanced with respect to natural vision by virtue of the use of an imaging sensor. The pilot therefore has real-time information of the outside. EVS increases visibility at night and in bad weather but, in the latter case, its effectiveness is limited and variable as a function of the types of fog and types of sensors used. Its use is therefore relevant above all when one is relatively close to the terrain.
The objective of CVS imaging is to best exploit the above two systems by combining them. The functions expected of CVS are based on those of EVS and of SVS taken individually, together with a bonus afforded by a relevant combination. These functions hinge around two features:
Improving the awareness of the situation of the crew in relation to the terrain, relevant obstacles and cultural elements, which may include towns, roads, rivers, helipads, runways, the environment of the airport, etc. thus offering the ability to continue a so-called “VFR” flight under reduced visibility at night or in bad weather;
Making up for the visual references required in “IFR” thus offering the aircraft the ability to descend under the minima authorized in the case of so-called “ILS CAT I” or “LPV” approaches on aerodrome or in the case of approaches on heliport of “Point in Space” type, or else in the case of off-shore approaches.
A first solution consists in overlaying the entire EVS image on the SVS image, thus masking a useful part of the SVS, optionally with registration of the SVS image on the EVS image by identifying a noteworthy element such as a landing runway. This representation necessarily limits the cases of use. It is illustrated in FIG. 1. In this figure, the SVS image is represented in wire-form and the EVS image as dotted zones. The symbology S is represented by simple geometric figures. This basic solution consisting in displaying the entire EVS image without transparency on the wider-field SVS image is not very satisfactory since the EVS image masks the useful information of the SVS image for all the parts representing distant terrain which the sensor does not penetrate.
A second possible scheme consists in proposing a transition between the EVS image and the SVS image. The problem to be solved is then to find a solution which makes it possible to overlay one image on the other and to pass from one to the other while maximizing the added value of each system. Indeed, beyond a certain distance between the terrain and the aircraft, which depends on the weather conditions and also on the air regulation applicable to the flight, the SVS image must then be predominant since the visibility of the EVS sensor does not make it possible to display a utilizable image to the crew of the aircraft. Conversely, below a certain distance from the terrain, the EVS image must be predominant since the SVS image may be a source of errors due to the inaccuracy of the location of the craft and of the databases.
Various possible criteria and various forms of transition exist. A first solution consists in displaying the EVS image only below the horizon and displaying the SVS image only above. A variant to this solution is described in Honeywell patent application US20120026190. The rendition of the “EVS” image on the “SVS” image is carried out with a first colour and a first transparency above the horizon and a second colour and a second transparency below the horizon. This all or nothing solution does not always best exploit the potential of the two images.
This solution is less brutal than the total overlaying of the EVS image on the SVS image. It has its limits however. In poor visibility due to fog or to snow, for example, the sensor does not penetrate as far as the horizon. Some useful information of the SVS may thus be masked through absence of transparency. This solution may also introduce confusion on the parts close to the terrain since the inaccuracy of the location and/or of the data arising from the databases may lead to a double display with shifting of the position of certain elements of the terrain or of obstacles or of landing runways.
A second type of transition solution is based on the analysis of the images themselves. A first variant consists in detecting the zones having a contrast greater than a given threshold in the EVS image and overlaying only these high-contrast zones on the SVS image. Patent application US2010283782 describes a fusion of images associating different colours or textures according to the type of SVS and EVS data. Patent application US2011227944 describes a fusion of SVS and EVS images after filtering of the EVS image by intensity threshold or frequency threshold, the two images being able to be distinguished by different formats and colours. U.S. Pat. No. 7,605,719 describes a solution which is much the same. Replacing the non-useful zones of the sensor's image with SVS imaging can render the CVS display confused. Indeed, the various replaced zones are not adjoining, the EVS image then exhibits “holes”. It is no longer possible to distinguish what arises from the image of the tracker or what arises from the synthetic image, and this may make it difficult for the pilot to interpret the resulting image.
Another type of solution consists in analysing the “semantic” content of the image. Thus, patent application US2008180351 describes a method for enhancing the EVS image around a point of interest that is known through the SVS database. Patent application US2012035789 employs this principle, applying it specifically to the approach ramps. Patent application US2010113149 describes the display in an SVS image of portions of images of one or more sensors representing a noteworthy element such as a landing runway or a fixed or mobile obstacle. U.S. Pat. No. 7,605,719 describes the detection of the useful and non-useful zones of the image arising from the tracker and the replacing of the non-useful zones with synthetic terrain, without giving more details.
The local enhancement of the EVS image or cropping around a point of interest known from the database operates only if there is actually a point of interest overflown and stored in database, typically a landing runway. This solution is not always satisfactory, for example, during low-altitude flights, typical of a helicopter mission where the helicopter flies constantly close to the ground without, however, frequently overflying points of interest stored in a database.
Another solution consists in determining by various means a visibility distance and in computing, as a function of this distance, the boundary separating the two images, SVS and EVS. Thus, patent FR2996670 claims the partial masking of an SVS image by the computation of the intersection of a sphere centred on the aeroplane or of a plane perpendicular to the aeroplane axis with the terrain, as a function of a visibility distance computed automatically or advised by the operator. Patent FR2996671 describes the partial masking of an SVS image around the landing runway and in a, wider, zone at the front of the runway whose length depends on the so-called “DH/A” approach procedure, the acronym standing for “Decision Height/Altitude”. In these two patents, the masked zone serves to display the sensor image.
This type of solution introduces an abrupt break between the EVS image and the SVS image which is not necessarily appreciated by pilots since the range limit of the sensor in poor visibility is not sharp and constant. Moreover, this solution requires the knowledge of the value of the visibility distance and may not be modified simply, for example, if the visibility distance changes over time.
Finally, it is also possible to separate the image arising from the tracker into three rectangular distinct zones. A first zone, at the bottom of the image is totally or almost opaque, the zone at the top of the image is totally or almost transparent, and the transition zone comprises a vertical linear opacity gradation. The drawback of this solution is that the boundary between the useful and non-useful zones is not determined. Thus, this solution is not optimal when the visibility of the sensor's image does not correspond to the distribution of the zones. In certain cases, useful information arising from the SVS is masked by a part of the sensor's image which turns out to be unutilizable. Finally, this solution based on rectangular zones is less appropriate when flying close to a craggy relief since, in this case, the limit of visibility is no longer a straight line.